An internal combustion engine including a piston crank mechanism (hereinafter, referred to as the engine) is subjected to a force and a moment that excite translational and rotational vibrations of a power plant formed of the engine and a transmission (hereinafter, referred to as the exciting force and the exciting moment), due to the inertia forces of a reciprocating mass such as the piston and a rotating mass such as the crankshaft, and also the gas pressure inside the cylinder (hereinafter, referred to as the cylinder internal pressure).
Here, of the frequency components of each of the exciting force and the exciting moment generated, the dominant component is determined by the cylinder arrangement of the engine. For example, in the case of inline four, the 2nd-order component of the engine revolution is the dominant component. This is because the crankshaft has pairs of crankpins located opposite to each other by 180 degrees in the case of inline four. A combustion stroke occurs twice while the crankshaft rotates once. Thus, combustion strokes occur at an interval of 180 degrees in terms of crank angle, and thus the dominant component of the rolling exciting moment is the 2nd-order component of the engine revolution which fluctuates twice per revolution. This is because when the exciting moments generated at all the cylinders are shifted by 180 degrees and combined, components with fluctuation cycles, integral multiples of which are 180 degrees, intensify each other and remain, whereas the other components (with cycles, ½ and ¼ of which are 180 degrees) cancel each other out. The 4th-, 6th-, . . . order components remain, but the 2nd-order component is the greatest. As another example, in the case of inline six, the 3rd-order component of the engine revolution is the dominant component due to the same reason. When a vehicle is equipped with a power plant as described above, anti-vibration support using rubber mounts or the like is employed in a vibration transmissive system for the purpose of improving vibration problems resulting from transmission of the power plant's vibrations derived to the vehicle.
As an apparatus for reducing vibrations due to the cylinder internal pressure of each cylinder, there is an apparatus which smoothens fluctuations in the rotational speed of each cylinder by detecting the fluctuations in the rotational speed of each cylinder in each expansion stroke and comparing it with the average value of all the rotational speed fluctuations (see Patent Document 1, for example). Moreover, there is an apparatus which controls the fuel injection amount of each cylinder based on a sound or vibration generated by combustion in the cylinder (see Patent Document 2, for example). However, while these apparatuses can reduce the above-mentioned components with the cycles, ½ and ¼ of which are 180 degrees, in inline four-cylinder engines by making even the torque generated by each cylinder, they cannot reduce the 2nd-order component. Moreover, the apparatuses require complicated control because the former apparatus controls the injection amount based on the fluctuations in rotational speed in each expansion stroke, and the latter apparatus controls the injection amount based on the sound or vibration of the cylinder.
Here, assuming a vibratory system in which a power plant is a mass and rubber mounts are springs and dampers, this vibratory system undergoes a resonance phenomenon (hereinafter, referred to as the mount resonance). FIG. 7 shows the relation between the ratio of a frequency ω of the exciting force or exciting moment to a mount resonance frequency ω0, and vibration transmissibility T from the engine side to the vehicle side. For anti-vibration support of a vehicular power plant, the mount resonance frequency ω0 is set such that the vibration transmissibility T falls within a region below 1 (hereinafter, anti-vibration region) in the case where the dominant component of a normal-operation rotational speed of the engine (an idling rotational speed or higher) is the frequency ω. Here, an anti-vibration region A0 is present in a range where ω/ω0 is above 1. Thus, in start and stop processes of the engine, a state of ω/ω0=1 (hereinafter, the resonance point RP1) is passed. As a result, the mount resonance causes a prominent vibrating phenomenon.
This vibrating phenomenon occurs in a power plant 1X including an engine 2x, a clutch housing 3, and a transmission 4 as shown in FIG. 8 and mounted on a vehicle with these apparatuses supported in such a way as to isolate vibrations by using rubber mounts 6. As shown in the figure, roll vibrations that shake the power plant 1X about the longitudinal axis of the engine 2X occur.
In a start process of an engine equipped with a start assist apparatus such as a starter motor, first, a torque is provided from the starter motor to perform intake and compression strokes. When the inside of a cylinder reaches an intake air state where combustion is possible, the combustion begins. Thereafter, the rotational speed is increased by using the torque generated by the engine itself through combustion therein and reaches a normal-operation rotational speed. The influence of the inertia force of the exciting moment generated in this process is small since the engine rotational speed is low, and the influence of the cylinder internal pressures is dominant. Here, in the above-mentioned mount resonance, the occurrence of roll vibrations excited by exciting moments due to the cylinder internal pressures is problematic in the case of a power plant in which roll resonance that shakes the power plant about the longitudinal axis of its engine is prominent.
FIG. 9 shows changes in the cylinder internal pressure of each cylinder in a start process of this engine. This figure shows the cylinder internal pressures of cylinders C1 to C4 when an inline four-cylinder engine is started. The order of ignition in this engine is the cylinder C2, the cylinder C1, the cylinder C3, and the cylinder C4. A region in which a torque is provided from the starter motor is a starter assist region A2, a region in which the rotational speed is increased by using the torque generated by the engine itself is a rotational-speed increasing region A3, and a region in which the engine rotational speed reaches a normal-operation rotational speed is a normal-operation-rotational-speed region A4. As also shown by the cylinder internal pressures, the rotational-speed increasing region A3 between 0.4 second to 0.9 second is within a process of increasing the engine rotational speed, and the cylinder internal pressures are high. The cylinder internal pressures then enter the normal-operation-rotational-speed region A4 and become substantially even. During the state where the cylinder internal pressures are high, there is a time point (a dotted line in the figure) t1 at which the above-mentioned resonance point RP1 is passed. The roll vibrations of the power plant occur due to the exciting moment resulting from the cylinder internal pressure of the cylinder C4 after the resonance point t1 that shows a high cylinder internal pressure.
Moreover, in the case where the operation has already reached the process of increasing the rotational speed by using the torque generated by the engine itself when the resonance point is passed, the exciting moment become large due to the increase in each cylinder internal pressure, thereby increasing the roll vibrations. On the other hand, it is desired to increase the cylinder internal pressures in view of improving startability such as successful start and a shorter start time. Thus, satisfying both startability and vibrations is a problem.